Certain vehicles can be driven using electrical power from one or more electric traction motors. For example, hybrid electric vehicles can selectively disconnect an internal combustion engine to allow vehicle propulsion in an electric-only/electric vehicle (EV) operating mode in order to conserve fuel, as well as to deliver immediate motor input torque to the transmission. A two-mode hybrid electric vehicle has a pair of EV modes to further optimize vehicle performance, as well as various fixed gear and electrically-variable transmission (EVT) modes.
In a strong hybrid design, the engine can be automatically restarted above a threshold speed, with engine torque used alone or in conjunction with motor torque from either or both of a pair of high-voltage electric traction motors to propel the vehicle. Other vehicle designs such as battery electric vehicles and extended-range electric vehicles also utilize EV operating modes to maximize fuel economy and minimize tailpipe emissions.
A vehicle with an EVT can be driven via a fraction motor when the engine is at zero speed (i.e., electrical drive), or the engine can be spinning while connected to the output with the vehicle at zero speed (i.e., engine-on launch), or anywhere in between. Typically, multiple hydraulically-actuated range clutches are used to allow shifting or transitioning between continuously-variable modes, hereinafter referred to as EVT modes, and multiple fixed-gear modes. In an input-split or series-parallel architecture, one traction motor may propel the vehicle while the other traction motor maintains the engine at zero speed. This helps to avoid losses from engine friction and pumping. However, single-motor electrical propulsion has limited electric launch and acceleration capabilities relative to the two-motor design of a two-mode hybrid electric vehicle.